Negative electrode active particle and method for manufacturing the same

ABSTRACT

A negative electrode active material particle and a method for preparing the same are provided. The negative electrode active material particle includes SiO x  (0&lt;x≤2) and Li 2 Si 2 O 5 , and includes less than 2 wt % of Li 2 SiO 3  and Li 4 SiO 4 .

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a Divisional of Application No. 15/333,525 filed onOct. 25, 2016, now U.S. Pat. No. 10,879,531, which claims the benefit ofKorean Patent Application No. 10-2015-0148807, filed on Oct. 26, 2015,in the Korean Intellectual Property Office, the entire contents of allof which are expressly incorporated by reference into the presentapplication.

TECHNICAL FIELD

The present disclosure relates to a negative electrode active materialparticle and a method for preparing the same.

BACKGROUND

Due to a rapid increase in the use of fossil fuels, there is a growingdemand for the use of alternative or clean energy. The fields which aremost actively being developed as part of this are those of electricalpower generation and energy storage.

Currently, secondary batteries are a representative example of anelectrochemical device that utilizes electrochemical energy, and theextent of the usage thereof is increasing. Recently, as technologicaladvancements and the demand for mobile devices such as mobile computers,mobile phones, cameras, etc. have increased, there has been a rapidincrease in demand for secondary batteries as sources of energy. Amongsuch secondary batteries, much research has been done on lithiumsecondary batteries, which exhibit high energy densities and operatingpotentials, and have long cycle lifetimes and low self-discharge rates,and having been commercialized, are in wide use.

As interest in environmental issues has increased, much research hasbeen devoted to electric vehicles, hybrid electric vehicles and the likewhich can replace fossil fuel powered vehicles such as gasoline poweredvehicles and diesel powered vehicles, which are among the leading causesof air pollution. Nickel-metal hydride batteries have been the mostwidely used power source for such electric vehicles, hybrid electricvehicles and the like, but lithium secondary batteries, which have highenergy densities and discharge voltages, are being actively researchedfor use in such vehicles, and have achieved a certain degree ofcommercialization.

Typically, lithium secondary batteries are composed of a positiveelectrode including a lithium transition metal oxide, a negativeelectrode including a carbon-based active material, and an electrolyte.Lithium ions released from a positive electrode active material areintercalated into a negative electrode active material such as a carbonparticle by an initial charge, and during discharge, are deintercalated.Energy transfer occurs as the lithium ions travel back and forth betweenthe two electrodes in this way, thus making charging/dischargingpossible.

Meanwhile, the carbon-based active material for negative electrodes hasexcellent stability and reversibility, but is limited in terms ofcapacity. Thus, in order to be used in mid- to large-scale energystorage systems and the like, the capacity must be at least two timesthe current capacity. Accordingly, a novel negative electrode activematerial is needed in order to achieve such mid- to large-scale energystorage systems.

Recently, negative electrode active materials capable of increasing thecapacity by a factor of four or more are being developed usingnon-carbon-based materials such as silicon (Si) and tin (Sn)

However, such materials have a limitation in which the volume of thenegative electrode and the secondary battery expand due to gas generatedafter a charge/discharge cycle, and thus are far from beingcommercialized.

DISCLOSURE OF THE INVENTION Technical Problem

Accordingly, an aspect of the present disclosure is to provide anon-carbon-based negative electrode active material particle which has ahigh capacity and may suppress the generation of gas caused by sidereactions.

Another aspect of the present disclosure is to provide a method forpreparing the negative electrode active material particle.

Another aspect of the present disclosure is to provide a negativeelectrode including the negative electrode active material particle, anda secondary battery including the same.

Technical Solution

According to an aspect of the present invention, there is provided anegative electrode active material particle which includes SiO_(x)(0<x≤2) and Li₂Si₂O₅, and which includes less than 2 wt % of Li₂SiO₃ andLi₄SiO₄.

The SiO_(x) (0<x≤2) may include a nanocomposite structure in which Siand SiO₂ are mixed therein.

In accordance with another aspect of the present disclosure, a methodfor preparing a negative electrode active material is provided. Themethod includes forming a lithium-silicon composite oxide by mixing asilicon oxide represented by SiO_(x) (0<x≤2) with a lithium precursorand then performing a first heat treatment (first step); and performinga second heat treatment on the lithium-silicon composite oxide (secondstep).

Here, the silicon oxide may include a nanocomposite structure in whichSi and SiO₂ are mixed therein.

In accordance with another aspect of the present disclosure, a negativeelectrode which comprises a negative electrode composition including thenegative electrode active material, and a secondary battery comprisingthe same are provided.

Advantageous Effects

A negative electrode active material according to the present disclosureincludes SiO_(x) (0<x≤2) and Li₂Si₂O₅, and includes less than 2 wt % ofLi₂SiO₃ and Li₄SiO₄. As a result, reactions between a non-aqueous binderincluded in the negative electrode active material and side products areprevented, and thus there is an effect of suppressing the generation ofgas from the negative electrode after charging/discharging. Therefore, alithium secondary battery having enhanced stability may be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the amount of gas generation according to timefor slurries including the negative electrode active materials inExample 2 and Comparative Examples 3 to 6 of the present disclosure.

DETAILED DESCRIPTION

Hereinafter, the present disclosure is described in greater detail tofacilitate understanding thereof.

The wordings or terms used in the specification and claims are notlimited to their typical or dictionary definitions. Rather wordings orterms as used herein are to be understood as defined by the inventor tobest convey the technical concepts of the present disclosure.

The terms used in the specification are merely used for describingexemplary embodiments, and are not intended to limit the presentdisclosure. Singular forms are intended to include the plural forms aswell, unless the context clearly indicates otherwise.

The terms “including”, “equipped with”, “having”, etc., when used in thespecification, specify the presence of stated features, integers,operations, elements, or combinations thereof, but do not preclude thepresence or addition of one or more other features, integers,operations, elements, or combinations thereof.

Specifically, an embodiment of the present disclosure provides anegative electrode active material particle which includes SiO_(x)(0<x≤2), Li₂Si₂O₅, Li₂SiO₃ and Li₄SiO₄, wherein Li₂SiO₃ and Li₄SiO₄ areincluded to be less than 2 wt %.

The SiO_(x) (0<x≤2) may include a nanocomposite structure in which Siand SiO₂ are mixed therein. Here, the composition (x) may be determinedby the ratio between silicon and oxygen. For example, when Si and SiO₂in the SiO_(x) (0<x≤2) are mixed in a 1:1 molar ratio, the SiO_(x) maybe represented as SiO_(x) where x=1.

Here, in the negative electrode active material of the presentdisclosure, the Si included in SiO_(x) (0<x≤2) is a component that cansubstantially cause an electrochemical reaction as lithium ions whichhave been deintercalated from a positive electrode active material areoccluded/released.

The Si may be crystalline or amorphous. When the Si is crystalline, theSi crystal grain size may be 500 nm or smaller, desirably 300 nm orsmaller, and more desirably 0.05 to 20 nm.

Here, the Si crystal grain size may be measured using x-ray diffraction(XRD) analysis or electron microscopy (SEM, TEM). Specifically, byperforming XRD (Bruker AXS D4 Endeavor XRD) analysis (voltage: 35 kV,current 28 mA, 20 range: 10° to 120°, step size: 0.019°, time per step:600 sec), the crystal grain size of the Si phase may be calculated viathe Scherrer equation using the full width at half maximum values of theSi peaks.

In the Si particle, the reaction in which lithium ions areelectrochemically occluded and released is accompanied by an extremelycomplex crystal change. For example, as the reaction in which lithiumions are electrochemically occluded and released proceeds, thecomposition or crystal structure of the Si particle changes to Si(crystal structure: Fd3m), LiSi (crystal structure: I41/a), Li₂Si(crystal structure: C2/m), Li₇Si₂ (Pbam), or Li₂₂Si₅ (F23) and the like.While going through such a Li—Si reaction process, the Si particle mayexpand in volume by a factor of four or more.

The negative electrode active material of the present disclosureincludes a component in which a silicon oxide represented by SiO_(x)(0<x≤2) has been doped beforehand with lithium ions such that instead ofthe Li—Si bonding reaction, lithium silicate (Li—Si—O) which is capableof reducing the initial non-reversibility is formed, thereby enhancinginitial efficiency while minimizing structural collapse of the negativeelectrode active material.

Moreover, in the negative electrode active material of the presentdisclosure, the SiO₂ included in the SiO_(x) (0<x≤2) may also becrystalline or amorphous. The crystalline SiO₂ may include quartz,cristobalite, or tridymite. When the SiO₂ is amorphous, it may beindicated as being included in the amorphous structure when analyzedusing XRD.

Li₂Si₂O₅ included in the negative electrode active material of thepresent disclosure is a product obtained by alloying a silicon oxidewith lithium, and less than 10 wt %, specifically 2 to 10 wt %, and morespecifically 2 to 5 wt % may be included with respect to the totalweight of the negative electrode active material. If 2 wt % or less ofthe Li₂Si₂O₅ is included, the amount is very small, and thus the initialefficiency-enhancing effect of alloying beforehand with Li isinsignificant. Conversely, when more than 10 wt % of the Li₂Si₂O₅ isincluded, there is an excess amount of an inactive phase, and thus anincrease in resistance may result in a decrease in the dischargecapacity per unit weight, and the negative electrode active material maybe in an unstable state.

Li₂SiO₃ and Li₄SiO₄ included in the negative electrode active materialof the present disclosure is an intermediate product generated whenlithiumizing silicon oxide to form Li₂Si₂O₅. When present in thenegative electrode active material, the Li₂SiO₃ and Li₄SiO₄ react withan aqueous binder and disadvantageously cause gas to be generated. Thus,it is desirable for 2 wt % or less—as measured by XRD—of the Li₂SiO₃ andLi₄SiO₄ to be included in the negative electrode active material, and inparticular, it is desirable that most of these components exist in theinterior of the negative electrode active material.

The Li₂SiO₃ and Li₄SiO₄ may be removed or remain in small amounts byundergoing a phase change induced by a simple heat treatment.Specifically, by mixing silicon oxide with a lithium precursor to form alithium-silicon composite oxide and then performing a heat treatingoperation at 700° C. or above, a phase change from Li₂SiO₃ and Li₄SiO₄to Li₂Si₂O₅ occurs to thereby decrease the concentration of Li₂SiO₃ andLi₄SiO₄ in the negative electrode active material to 2 wt % or lower. Inparticular, the Li₂SiO₃ and Li₄SiO₄ are mostly removed from the surfaceof the negative electrode active material. When more than 2 wt % remainsin the negative electrode active material, the Li₂SiO₃ and Li₄SiO₄ mayreact with an aqueous binder during charging/discharging, therebygenerating gas from the negative electrode.

Here, whether or not Li₂SiO₃ and Li₄SiO₄ remain in the negativeelectrode active material may be determined by XRD analysis using anx-ray diffractometer (Bruker AXS D4-Endeavor XRD).

Specifically, after mixing the prepared negative electrode materialparticle with MgO in a 8:2 weight ratio, XRD Rietveld analysis methodmay be used to measure the content ratios of the components Si,Li₂Si₂O₅, Li₂SiO₃, and Li₄SiO₄ in the negative electrode activematerial. Here, a voltage of 40 kV and a current of 40 mA were applied,the 20 range was 10° to 90°, and scanning was performed using a stepsize of 0.05°. The slit used was a variable divergence slit (6 mm), anda large PMMA holder (diameter=20 mm) was used to remove background noisedue to the PMMA holder.

The average particle diameter (D50) of the negative electrode activematerial may be 0.05 nm to 30 μm, specifically, 0.5 nm to 15 μm.

The average particle diameter (D50) of the negative electrode activematerial may be measured using a laser scattering method or anelectrical resistance method (Coulter counter method), and specifically,the laser scattering method was used.

The laser scattering method is a method which utilizes the simultaneousand combined effects of diffraction, refraction, reflection, andabsorption, wherein the particle size is derived based on the principlein which the scattering intensity is proportional to the particle sizeand the scattering angle is inversely proportional to the particle size.Particles passing through a laser beam scatter light, and the scatteredlight forms a different scattering pattern for each angle, which isanalyzed by being detected by a photodetector array. In such laserscattering methods, fine particles are beyond the detection range andthus are mostly undetected.

In the present disclosure, the measurement may be performed on aspecimen in the form of a dispersion formed by dispersing the activematerial in water, which is the solvent, and adding a surfactant asneeded.

As described above, in order to suppress the formation of anon-reversible lithium oxide phase during the initial charging processof the battery, in the negative electrode active material according tothe present disclosure, a component obtained by alloying a silicon oxiderepresented by SiO_(x) (0<x≤2) with lithium may be included to therebyincrease the initial efficiency. In particular, the content ofintermediate products—such as Li₂SiO₃ and Li₄SiO₄—produced while alloyof the negative electrode active material of the present invention withlithium is formed, is reduced by or most of such intermediate productsare removed by a heat treatment of two steps. Accordingly, thelimitation of the intermediate products reacting with an aqueous bindersuch that gas is generated may be overcome.

The negative electrode active material for a secondary battery of thepresent disclosure may further include—as needed—a carbon coating layeron the surface of the SiO_(x) (0<x≤2).

Moreover, the carbon coating layer may include a crystalline or anamorphous carbon coating layer. The crystalline carbon coating layer maybe formed by mixing the inorganic or inorganic oxide particle with acrystalline carbon in a solid or liquid phase and then heat treating.The amorphous carbon coating layer may be formed by using a method inwhich the inorganic or inorganic oxide particle is coated with anamorphous carbon precursor and then carbonized by being heat treated.

Here, representative examples of the crystalline carbon may includegraphene and graphite. Specific examples of the amorphous carbonprecursor may be any one selected from the group consisting of resins, acoal-based pitch, tar, and low molecular weight heavy oil. The examplesof resin may be a phenol resin, a naphthalene resin, a polyvinyl alcoholresin, a urethane resin, a polyimide resin, a furan resin, a celluloseresin, an epoxy resin, a polystyrene resin, and the like.

The carbon coating layer is commonly included in an amount of less than20 wt %, for example, 1 wt % to 10 wt % based on the total weight of thenegative electrode material particle.

Here, when the carbon coating layer exceeds 20 wt %, the thickness ofthe carbon coating layer is excessively thick such that lithiumintercalation and deintercalation are inhibited, and thus, the dischargecapacity is reduced and an initial efficiency may be reduced due to anon-reversible reaction between amorphous carbon and lithium.

Provided is a method for preparing a negative electrode activematerial—which can achieve such effects—according to an embodiment ofthe present disclosure. The method includes mixing a silicon oxiderepresented by SiO_(x) (0<x≤2) with a lithium precursor and thenperforming a first heat treatment to from a lithium-silicon compositeoxide (first step); and performing a second heat treatment on thelithium-silicon composite oxide (second step).

Hereinafter, each operation of a method for preparing a negativeelectrode active material according to the present disclosure will bedescribed in detail.

In the method for preparing a negative electrode active materialparticle according to the present disclosure, a first step is anoperation for forming a lithium-silicon composite oxide by mixing asilicon oxide represented by SiO_(x) (0<x≤2) with a lithium precursorand then performing a first heat treatment.

Here, the silicon oxide may be a nanocomposite structure in which Si andSiO₂ are mixed therein in an approximately 1:1 molar ratio, and mayinclude SiO.

Moreover, the lithium precursor may include a lithium powder or alithium salt, and specifically, at least one selected from the groupconsisting of Li powder, LiH, LiAlH, LiOH, Li₂CO₃, LiCl, Li₂O, and LiFmay be used.

The silicon oxide and the lithium precursor may be mixed in a 70:30 to97:3 weight ratio.

Here, when the mixing ratio by weight of the lithium precursor is lessthan 3, the anticipated effect of alloying beforehand with lithium ionsis slight, and when greater than 30, lithium and silicon excessivelyreact to form, in addition to the lithium-silicon composite oxide, alithium-silicon alloy. Consequently, the negative electrode activematerial may be unstable against the external atmosphere and moisture.

The first heat treatment may be performed for 4 to 6 hours at atemperature of 650 to 750° C.

During the first heat treatment, Li₂Si₂O₅ which is a lithium-siliconoxide, and Li₂SiO₃ and Li₄SiO₄, which are intermediate products, may beproduced.

If the first heat treatment is performed at a temperature below 650° C.or for less than 4 hours, the alloying with lithium may not be desirablyperformed, and thus Li₂SiO₃, Li₄SiO₄, and Li₂Si₂O₅ may not be formed.Conversely, when the first heat treatment is performed at a temperatureabove 750° C. or for longer than 6 hours, the alloying with lithium maybe excessive, and thus Si capacity may be reduced.

Meanwhile, the method of the present disclosure may further include—asneeded—forming a carbon coating layer on the surface of the siliconoxide before mixing the silicon oxide with the lithium precursor.

Here, the operation for forming the carbon coating layer may typicallybe performed using a CVD method using soft carbon, hard carbon,graphene, amorphous carbon, crystalline carbon (graphite), etc., or apitch coating method

Here, the carbon coating layer is commonly included in an amount of lessthan 20 wt %, for example, 1 wt % to 10 wt % based on the total weightof the negative electrode material particle.

When the carbon-coated silicon oxide is mixed with the lithiumprecursor, a portion of the lithium-silicon oxide is also present in theformed carbon coating layer, and the reactivity with water is determinedby the type of the lithium-silicon composite oxide. Here, when thecarbon coating layer exceeds 20 wt %, the thickness of the carboncoating layer is excessively thick such that lithium intercalation anddeintercalation are inhibited, and thus the discharge capacity isreduced and an initial efficiency may be reduced due to a non-reversiblereaction between amorphous carbon and lithium.

In the method for preparing a negative electrode active materialaccording to the present disclosure, a second step is an operation forinducing a phase transformation of the Li₂SiO₃, and Li₄SiO₄—producedwhen forming the lithium-silicon composite oxide—into Li₂Si₂O₅.

The second heat treatment operation may be performed at a temperature of700 to 1,100° C., specifically 900 to 1,100° C., for 1 to 3 hours,specifically, 1 to 2 hours.

Since most of the Li₂SiO₃ and Li₄SiO₄—intermediate products produced inaddition to the final product Li₂Si₂O₅—undergoes a phase transformationto Li₂Si₂O₅, the Li₂SiO₃ and Li₄SiO₄ may be reduced in content or almostentirely removed in the negative electrode active material by the secondheat treatment. In particular, as a result of the second heat treatment,Li₂SiO₃ and Li₄SiO₄ may remain in small amounts in the interior of thenegative electrode active material, and be nearly completely removedfrom the surface.

Here, when the second heat treatment operation is performed at atemperature below 700° C. or for less than 1 hour, the phasetransformation effect of Li₂SiO₃, Li₄SiO₄ is very slight. When thesecond heat treatment operation is preformed at a temperature above1,100° C. of for longer than 3 hours, SiO₂ and Si phases grow separatelyin the interior of the prepared negative electrode active materialparticle, and thus a volume control effect may be reduced.

An embodiment of the present disclosure provides a negative electrodeincluding a current collector and a negative electrode compositionincluding the negative electrode active material particle, formed on atleast one surface of the current collector.

Specifically, the negative electrode composition may be prepared bymixing the negative electrode active material of the present disclosure,a conductive material, and the binder in a solvent to prepare a negativeelectrode material slurry composition, applying the slurry onto thenegative electrode current collector, and then drying and roll-pressing.

Here, since there is almost no Li₂SiO₃ and Li₄SiO₄ present in thenegative electrode active material, or if present, a very small amount,the typical problem of the aqueous binder reacting with Li₂SiO₃ andLi₄SiO₄ to generate a gas may be overcome.

The negative electrode active material may be included in an amount of80 wt % to 99 wt % based on the total weight of the negative electrodeactive material slurry composition.

The binder is a component which facilitates the bonding between theconductive material and the active material or current collector, andtypically, may be included in an amount of 0.1 wt % to 20 wt % based onthe total weight of the negative electrode active material slurrycomposition. Examples of the binder may include polyvinylidene fluoride(PVdF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch,hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone,tetrafluoroethylene, polyethylene, polypropylene,ethylene-propylene-diene monomer (EPDM), sulfonated-EPDM, styrenebutadiene rubber (SBR), fluorocarbon rubber, and various copolymersthereof, and specifically, the binder may include an aqueous bindercomposed of carboxymethyl cellulose (CMC), styrene butadiene rubber(SBR), or a mixture thereof.

The conductive material may be used without particular limitation solong as it has suitable conductivity without causing adverse chemicalchanges in the battery, and is commonly added in an amount of 1 wt % to20 wt % based on the total weight of the negative electrode materialslurry composition. Any conductive agent may be used without particularlimitation so long as it has suitable conductivity without causingadverse chemical changes in the battery, and, for example, a graphitesuch as a natural graphite or a synthetic graphite; a carbon black suchas acetylene black, Ketjen black, channel black, furnace black, lampblack, or thermal black; a carbon-based material such as carbon black,acetylene black, Ketjen black, channel black, furnace black, lamp black,and thermal black; a conductive fiber such as a carbon fiber or a metalfiber; a fluorocarbon; a metal powder such as aluminum or nickel powder;a conductive whisker such as zinc oxide or potassium titanate; aconductive metal oxide such as titanium oxide; or a conductive materialsuch as a polyphenylene derivative.

The solvent may include water or an organic solvent such asn-methyl-2-pyrrolidone (NMP), and may be used in an amount such thatdesirable viscosity is obtained when the negative electrode activematerial as well as selectively the binder and the conductive agent areincluded. For example, the solvent may be included so that aconcentration of solid content including the negative electrode activematerial as well as selectively the binder and the conductive agent isin a range of 50 wt % to 95 wt %, for example, 70 wt % to 90 wt %.

An embodiment of the present disclosure provides a secondary batterywhich includes a positive electrode, a negative electrode, and anelectrolyte solution, wherein the negative electrode is the negativeelectrode of the present disclosure.

The positive electrode may be prepared by coating a positive electrodematerial slurry composition including a positive electrode activematerial, a binder, a conductive agent, and a solvent onto a positiveelectrode current collector, and then drying and roll-pressing.

The positive electrode active material is a compound capable ofreversibly intercalating and deintercalating lithium, wherein thepositive electrode active material may specifically include a lithiumcomposite metal oxide including lithium and at least one metal such ascobalt, manganese, nickel, or aluminum. Specifically, the lithiumcomposite metal oxide may include lithium-manganese-based oxide (e.g.,LiMnO₂, LiMn₂O₄, etc.), lithium-cobalt-based oxide (e.g., LiCoO₂, etc.),lithium-nickel-based oxide (e.g., LiNiO₂, etc.),lithium-nickel-manganese-based oxide (e.g., LiNi_(1-Y)Mn_(Y)O₂ (where0<Y<1), LiMn_(2-Z)Ni_(Z)O₄ (where 0<Z<2), etc.),lithium-nickel-cobalt-based oxide (e.g., LiNi_(1-Y1)Co_(Y1)O₂ (where0<Y1<1), lithium-manganese-cobalt-based oxide (e.g.,LiCo_(1-Y2)Mn_(Y2)O₂ (where 0<Y2<1), LiMn_(2-Z1)Co_(Z1)O₄ (where0<Z1<2), etc.), lithium-nickel-manganese-cobalt-based oxide (e.g.,Li(Ni_(p)Co_(Q)Mn_(R))O₂ (where 0<P<1, 0<Q<1, 0<R<1, and P+Q+R=1) orLi(Ni_(P1)Co_(Q1)Mn_(R1))O₄ (where 0<P1<2, 0<Q1<2, 0<R1<2, andP1+Q1+R1=2), etc.), or lithium-nickel-cobalt-transition metal (M) oxide(e.g., Li(Ni_(P2)Co_(Q2)Mn_(R2)M_(S2))O₂ (where M is selected from thegroup consisting of aluminum (Al), iron (Fe), vanadium (V), chromium(Cr), titanium (Ti), tantalum (Ta), magnesium (Mg), and molybdenum (Mo),and P2, Q2, R2 and S2 are atomic fractions of each independent elements,wherein 0<P2<1, 0<Q2<1, 0<R2<1, 0<S2<1, and P2+Q2+R2+S2=1), etc.), andany one thereof or a compound of two or more thereof may be included.Among these materials, in terms of the improvement of the capacitycharacteristics and stability of the battery, the lithium compositemetal oxide may include LiCoO₂, LiMnO₂, LiNiO₂, lithium nickel manganesecobalt oxide (e.g., Li(Ni_(0.6) Mn_(0.2) Co_(0.2))O₂,Li(Ni_(0.5)Mn_(0.3)Co_(0.2))O₂, Li(Ni_(0.7)Mn_(0.15)Co_(0.15))O₂, orLi(Ni_(0.8)Mn_(0.1)Co_(0.1))O₂), or lithium nickel cobalt aluminum oxide(e.g., LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, etc.). In consideration of asignificant improvement due to the control of type and content ratio ofelements constituting the lithium composite metal oxide, the lithiumcomposite metal oxide may include Li(Ni_(0.6)Mn_(0.2)Co_(0.2))O₂,Li(Ni_(0.5)Mn_(0.3)Co_(0.2))O₂, Li(Ni_(0.7)Mn_(0.15)Co_(0.15))O₂, orLi(Ni_(0.8)Mn_(0.1)Co_(0.1))O₂, and any one thereof or a mixture of twoor more thereof may be used.

The positive electrode active material may be included in an amount of80 to 99 wt % based on a total weight of the positive electrode materialslurry composition.

Meanwhile, the binder for the positive electrode is a component thatassists in the binding between the active material and the conductiveagent and in the binding with the current collector, wherein the binderis commonly added in an amount of 1 wt % to 20 wt % based on the totalweight of the positive electrode material slurry composition. Varioustypes of binder polymers may be used, including polyvinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol,carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose,regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene,polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-dienemonomer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR),fluorocarbon rubber, polyacrylic acid, polymers in which the hydrogenatoms thereof are substituted with Li, Na, or Ca, or various copolymersthereof.

Any conductive agent may be used without particular limitation so longas it has suitable conductivity without causing adverse chemical changesin the battery, and, is commonly added in an amount of 1 wt % to 20 wt %based on the total weight of the positive electrode material slurrycomposition. Any conductive agent may be used without particularlimitation so long as it has suitable conductivity without causingadverse chemical changes in the battery, and, for example, a graphitesuch as a natural graphite or a synthetic graphite; a carbon black suchas acetylene black, Ketjen black, channel black, furnace black, lampblack, or thermal black; a carbon-based material such as carbon black,acetylene black, Ketjen black, channel black, furnace black, lamp black,and thermal black; a conductive fiber such as a carbon fiber or a metalfiber; a fluorocarbon; a metal powder such as aluminum or nickel powder;a conductive whisker such as zinc oxide or potassium titanate; aconductive metal oxide such as titanium oxide; or a conductive materialsuch as a polyphenylene derivative.

The solvent may include water or an organic solvent such asn-methyl-2-pyrrolidone (NMP), and may be used in an amount such thatdesirable viscosity is obtained when the positive electrode activematerial as well as selectively the binder and the conductive agent areincluded. For example, the solvent may be included so that aconcentration of solid content including the positive electrode activematerial as well as selectively the binder and the conductive agent isin a range of 50 wt % to 95 wt %, for example, 70 wt % to 90 wt %.

The electrolyte solution may include a non-aqueous organic solvent and ametal salt.

An aprotic organic solvent may be used as the non-aqueous organicsolvent, for example, n-methyl-2-pyrollidone, propylene carbonate,ethylene carbonate, butylene carbonate, dimethyl carbonate, diethylcarbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran,2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide,dimethylformamide, dioxolane, acetonitrile, nitromethane, methylformate, methyl acetate, triphosphate ester, trimethoxymethane, adioxolane derivative, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidone, a propylene carbonate derivative, atetrahydrofuran derivative, ether, methyl propionate, or ethylpropionate.

A lithium salt may be used as the metal salt, and a material that easilydissolves in the non-aqueous solution may be used as the lithium salt,for example, LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃,LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi,etc.

The positive electrode and negative electrode current collector havingconductivity and not chemically changing the battery is not particularlylimited. For example, copper, stainless steel, aluminum, nickel,titanium, baked carbon, or aluminum or stainless steel surface treatedwith carbon, nickel, titanium, or silver and the like may be used.

According to another embodiment of the present disclosure, a batterymodule including the secondary battery as a unit cell, and a batterypack including the same are provided. The battery module and the batterypack include a secondary battery having a high capacity, a longlifetime, and a high initial efficiency, and may be used as a powersource for a mid- to large-scale device selected from the groupconsisting of electric vehicles, hybrid electric vehicles, plug-inhybrid electric vehicles, and electrical energy storage systems.

Hereinafter, examples of the present disclosure are described in detailin order to allow the present disclosure to be carried out by a personwith ordinary skill in the art. However, the present disclosure may beembodied in various forms and is not limited to the examples describedherein.

EXAMPLES Example 1

(First Step)

A mixture was formed by mixing a silicon oxide (SiO_(x) that is Si andSiO₂ are mixed in a 1:1 molar ratio in SiO_(x) (0<x≤2)) having anaverage particle diameter of 4 μm with a lithium metal powder (Li) in a97:3 weight ratio. A lithium-silicon composite oxide including SiO alongwith Li₂Si₂O₅, Li₂SiO₃, and Li₄SiO₄ was formed by performing a firstheat treatment on the mixture in an argon atmosphere at 700° C. for 5hours.

(Second Step)

A negative electrode active material particle was prepared by performinga second heat treatment in an argon atmosphere at 900° C. for 2 hours onthe lithium-silicon composite oxide formed above.

After mixing the prepared active material particle in a 8:2 weight ratiowith MgO, which has excellent crystallinity and does not have XRD peaksthat overlap with the active material, XRD Rietveld analysis method isperformed by an x-ray diffractometer (Bruker AXS D4-Endeavor XRD). Onthe basis of a graph obtained from the XRD Rietveld analysis method, thecontent ratio of each of the components—Si, amorphous structure (SiO₂,etc.), Li₂Si₂O₅, Li₂SiO₃ and Li₄SiO₄-present in the prepared negativeelectrode active material particle was measured, and the results aredisplayed in Table 1.

Specifically, after mixing the prepared active material particle withMgO in a 8:2 weight ratio, the content ratios of Si, Li₂Si₂O₅, Li₂SiO₃,and Li₄SiO₄ present in the negative electrode active material particlecould be measured using the Rietveld analysis method. Specifically, avoltage of 40 kV and a current of 40 mA were applied, the 2θ range was10° to 90°, and scanning was performed using a step size of 0.05°. Here,the slit used was a variable divergence slit (6 mm), and a large PMMAholder (diameter=20 mm) was used to remove background noise due to thePMMA holder.

Comparative Example 1

(First Step)

A mixture was formed by mixing a silicon oxide (SiO_(x) (0.5≤x≤1.5)having an average particle diameter (D50) of 4 μm with a lithium metalpowder (Li) in a 97:3 weight ratio. A lithium-silicon composite oxideincluding SiO along with Li₂Si₂O₅, Li₂SiO₃, and Li₄SiO₄ was formed byheat treating the mixture in an argon atmosphere at 700° C. for 5 hours.

Using the XRD Rietveld analysis method, the content ratio of each of thecomponents—Si, amorphous structure (SiO₂, etc.), Li₂Si₂O₅, Li₂SiO₃, andLi₄SiO₄— present in the negative electrode active material particle wasmeasured, and the results are displayed in Table 1

Comparative Example 2

A mixture was formed by mixing a silicon oxide (SiO_(x) (0.5≤x≤1.5)having an average particle diameter (D50) of 4 μm with a lithium metalpowder (Li) in a 97:3 weight ratio. A lithium-silicon composite oxideincluding SiO along with Li₂Si₂O₅, Li₂SiO₃, and Li₄SiO₄ was formed byheat treating the mixture in an argon atmosphere at 750° C. for 5 hours.

By treating the above-formed lithium-silicon composite oxide with anacidic aqueous solution, a negative electrode active material wasprepared. Using the XRD Rietveld analysis method, the content ratio ofeach of the components—Si, amorphous structure (SiO₂, etc.), Li₂Si₂O₅,Li₂SiO₃, and Li₄SiO₄—present in the prepared negative electrode activematerial particle was measured, and the results are displayed in Table1.

TABLE 1 Component ratio (wt %) Amorphous Si Li₂SiO₃ Li₄SiO₄ Li₂Si₂O₅structure Example 1 27 wt %  2 wt % 0 14 wt % 57 wt % Comparative 26 wt% 12 wt % 3 wt %  3 wt % 56 wt % Example 1 Comparative 26 wt % 14 wt % 0 4 wt % 56 wt % Example 2

As illustrated in Table 1, it may be confirmed that the negativeelectrode active material particle of Examples 1 has small amounts-2 wt% and 0 wt % respectively—of Li₂SiO₃ and Li₄SiO₄ remaining, but thenegative electrode active material particles prepared in ComparativeExamples 1 and 2 have a Li₂Si₂O₅ concentration that is actually lowerthan the Li₂SiO₃ and Li₄SiO₄ content.

That is, in the case of the negative electrode active particle ofComparative Example 1, on which a second heat treatment was notperformed, Li₂SiO₃ and Li₄SiO₄ as well as Li₂Si₂O₅ are observed. Here,while large amounts of Li₂SiO₃ and Li₄SiO₄— 12 wt % and 3 wt %,respectively are detected, it may be seen that the concentration ofLi₂Si₂O₅ is 3 wt %, which is equal to or significantly lower than thoseof the intermediate products, Li₂SiO₃ and Li₄SiO₄

In addition, when, as in Comparative Example 2, acid treatment ratherthan the second heat treatment is performed, Li₄SiO₄ is completelyremoved (0 wt %), whereas it is difficult to remove Li₂SiO₃, and thus itmay be seen that 14 wt % of Li₂SiO₃ which is greater than Li₂Si₂O₅ (4 wt%) is detected. Thus, it can be anticipated that a mitigating effect ofgas generation will be low in the negative electrode active materialprepared according to the method of Comparative Example 2.

Example 2

(Manufacturing Negative Electrode)

A mixture was formed by mixing the negative electrode active materialparticle prepared in Example 1, a conductive material, carbon black, anda binder, SBR/CMC, in a weight ratio of 96:2:2. The mixture was placedin distilled water as a solvent and stirred for 60 minutes to prepare anegative electrode active material slurry composition having a solidmaterial content of 85 wt %.

Using a doctor blade, the negative electrode active material slurrycomposition was applied to a thickness of about 60 μm on a coppercurrent collector having a thickness of 10 μm, and after being dried for0.5 hours at 100° C. in a hot-air dryer, was dried once more for 4 hoursat 120° C. in a vacuum and roll-pressed to manufacture a negativeelectrode plate.

(Manufacturing Positive Electrode)

After introducing 97.45 wt % of LiNi_(0.6)Mn_(0.2)Co_(0.2)O₂, 0.5 wt %of synthetic graphite (SFG6, Timcal) powder as a conductive material,0.7 wt % of carbon black (Ketjenblack, ECP), 1.15 wt % of polyvinylidenefluoride (PVDF, S6020, Solvay), and 0.2 wt % of polyvinylidene fluoride(PVDF, S5130, Solvay) in a n-methyl-2-pyrollidone solvent, a positiveelectrode active material slurry composition having a solid materialcontent of 85 wt % was prepared by stirring for 30 minutes with amechanical stirrer.

Using a doctor blade, the positive electrode active material slurrycomposition was applied to a thickness of 60 μm on an aluminum currentcollector having a thickness of 20 μm, and after being dried for 0.5hours at 100° C. in a hot-air dryer, was dried once more for 4 hours at120° C. in a vacuum and roll-pressed to manufacture a positive electrodeplate.

(Manufacturing Secondary Battery)

An electrolyte solution for a secondary battery was manufactured bydissolving 1.0 M of LiPF₆ in a non-aqueous organic solvent composed ofethylene carbonate/ethyl methyl carbonate/diethyl carbonate(EC:EMC:DEC=3:5:2 by vol %).

An electrode assembly was manufactured by interposing a porouspolyethylene separator between the manufactured negative and positiveelectrodes, and after disposing the electrode assembly in a case for asecondary battery, a lithium secondary battery was manufactured byinjecting the electrolyte solution into the case.

Comparative Example 3

A negative electrode active material slurry composition, and a negativeelectrode and secondary battery including the same were prepared in thesame manner as in Example 2 except that the negative electrode activematerial particle prepared in Comparative Example 1, instead of thenegative electrode active material particle prepared in Example 1, wasincluded.

Comparative Example 4

A negative electrode active material slurry composition, and a negativeelectrode and secondary battery including the same were prepared in thesame manner as in Example 2 except that the negative electrode activematerial particle prepared in Comparative Example 2, instead of thenegative electrode active material particle prepared in Example 1, wasincluded.

Comparative Example 5

A negative electrode active material slurry composition, and a negativeelectrode and secondary battery including the same were prepared in thesame manner as in Example 2 except that Li₂SiO₃ particle, instead of thenegative electrode active material particle prepared in Example 1, wasincluded as the negative electrode active material particle.

Comparative Example 6

A negative electrode active material slurry composition, and a negativeelectrode and secondary battery including the same were prepared in thesame manner as in Example 2 except that Li₄SiO₄ particle, instead of thenegative electrode active material particle prepared in Example 1, wasincluded as the negative electrode active material particle.

Experimental Example Experimental Example 1

Each of the negative electrode active material slurries (4 g) preparedin Example 2 and Comparative Examples 3 to 6 was placed in a stainlesssteel container having a pressure sensor attached thereto, and aftercutting off external air, the increased pressure due to gas generatedinside the container was measured by the pressure sensor, and theresults thereof are displayed in Table 2 and FIG. 1.

TABLE 2 Time at which gas was Material generated (s) Pressure (bar)Example 2 >50,000 <0.05 Comparative 12,000 0.65 Example 3 Comparative12,000 0.62 Example 4 Comparative 8,700 0.69 Example 5 Comparative 6,5000.73 Example 6

As shown in Table 2 and FIG. 1, in the case of the negative electrodeactive material slurry composition in Example 2, even after an amount oftime has passed, the pressure due to gas was less than 0.05 bar, whichis an insignificant level.

In Contrast, in the case of Comparative Examples 3 and 4, it may be seenthat as the pressure increased rapidly after about 12,000 seconds, alarge amount of gas was generated—at least about 90% more than inExample 2.

In particular, from Table 2, it may be seen that in the case ofComparative Examples 3 and 4, in which the second heat treatment was notperformed, the effect of removing Li₂SiO₃ and Li₄SiO₄ was slight, andthus an excessive amount of gas was generated, while in the case ofExample 1, in which the second heat treatment was performed, most of theLi₂SiO₃ and Li₄SiO₄ underwent a phase transformation to Li₂Si₂O₅, andthus gas generation was suppressed.

As in Comparative Examples 5 and 6, it may be seen that a large amountof gas was generated from the negative electrode active material slurrycomposition including Li₂SiO₃ and Li₄SiO₄.

Although exemplary embodiments of the present disclosure have beendescribed, the present disclosure is not limited thereto. Rather, it isunderstood that various changes and modifications made by a personskilled in the art using the basic concepts of the disclosure as definedin the claims are within the scope of the disclosure.

What is claimed is:
 1. A method for preparing a negative electrodeactive material particle, the method comprising: mixing a silicon oxiderepresented by SiO_(x) (0<x≤2) with a lithium precursor and thenperforming a first heat treatment to form a lithium-silicon compositeoxide(first step); and performing a second heat treatment on thelithium-silicon composite oxide (second step), wherein the first heattreatment is performed for 4 to 6 hours at a temperature of 650° C. to750° C., and wherein the second heat treatment is performed for 1 to 3hours at a temperature of 900° C. to 1,100° C.
 2. The method of claim 1,wherein the lithium precursor is at least one selected from the groupconsisting of lithium metal powder and lithium salt.
 3. The method ofclaim 2, wherein the lithium precursor is at least one selected from thegroup consisting of Li metal powder, LiOH, Li₂CO₃, LiCl, Li₂O, and LiF.4. The method of claim 1, wherein the silicon oxide and the lithiumprecursor are mixed in a 70:30 to 97:3 weight ratio.
 5. The method ofclaim 1, further comprising step for forming a carbon coating layer on asurface of the silicon oxide before mixing the silicon oxide with thelithium precursor.
 6. The method of claim 5, wherein the carbon coatinglayer is formed in an amount of 1 wt % to 10 wt % based on a totalweight of the negative electrode material particle.
 7. The method ofclaim 1, wherein the negative electrode active material particlecomprises SiO_(x) (0<x≤2); and Li₂ Si₂O₅, which includes less than 2 wt% of Li₂SiO₃ and Li₄SiO₄.
 8. The method of claim 1, wherein the SiO_(x)(0<x≤2) includes SiO.
 9. The method of claim 8, wherein the SiO_(x)(0<x≤2) includes a nanocomposite structure in which Si and SiO₂ aremixed in a 1:1 molar ratio.